The present invention relates generally to contactors, and more particularly to a method and apparatus to prevent contacts from welding shut after a fault condition in an electromagnetic contactor.
In some applications, particularly in electromechanical motor controllers, a short circuit fault current condition generates an extremely high constriction force across the contact surfaces in a contactor. Such high constriction forces often overcomes the contact biasing forces and leads to the blow open of the contacts. Because of the rapid decrease of arc pressure difference across the movable contacts after the contacts are blown open, together with the increasing force created by the biasing spring when further compressed, the contacts will re-close within a few milliseconds, and usually well before the fault current has returned to current zero which can result in the permanent welding of the contacts. In other words, contact separation under short circuit conditions results routinely in an arcing between the movable and fixed contacts. This arcing can cause the contacts to melt on a momentary separation incident to the short circuit and if the contacts were to close together before the molten metal cools and solidifies, the fixed and movable contacts will become firmly and permanently welded together. Such welding can happen in a very short time interval due to the high current flow of the short circuit blowing open the contacts, which are then almost instantaneously forced closed by the reaction of the contact biasing spring.
In conventional contactors, no special means is provided to prevent blow open at short circuit fault currents except for the contact biasing springs. In an effort to overcome the effect of contact blow open, the typical approach is to use the magnetic force induced by the short circuit fault to keep the contacts closed during the high current. One example of such a system is disclosed in U.S. Pat. No. 3,887,888 in which a pair of magnetic members surround the contacts whereby on occurrence of a short circuit through the contacts, the magnetic members are attracted to one another thereby forcing the contacts together. Similarly, U.S. Pat. No. 4,513,270 uses the magnetic flux developed in a magnetic member when an overload current flows through a contactor generating electrodynamic forces to force the movable contacts against the stationary contacts so as to prevent the contacts from separating.
One disadvantage of attempting to keep the contacts closed during a short circuit is that such an approach is limited by either the magnetic saturation of the magnetic components that generate the force, or by a complex design of the current path resulting in an increased cost of the contactors. This problem is exaggerated when the FLA rating of a contactor is below 125 amps since current limiting circuit breakers have little protection below 10,000 amps.
Therefore, it would be desirable to have a method and apparatus that could prevent contact welding under fault conditions by opening the contacts relatively quickly upon the occurrence of a fault condition and maintaining the contacts open until the fault condition dissipates, thereby allowing the contact surfaces to cool sufficiently and ensure contact solidification before closure to allow closure without subsequent welding.